1. Field of the Invention
This invention relates to flexible, longlife, conductive polymeric material for current-limiting applications with circuit breakers and other electrical apparatus.
2. Background Information
Current limiting polymer compositions which exhibit positive temperature coefficient of resistance (PTC) behavior, and electrical devices comprising current limiting polymer compositions have been widely used. The current limiting polymer compositions generally include conductive particles, such as carbon black, graphite or metal particles, dispersed in a polymer matrix, such as thermoplastic polymer, elastomeric polymer or thermosetting polymer. PTC behavior in a current limiting polymer composition is characterized by the material undergoing a sharp increase in resistivity as its temperature rises above a particular value known as the switching temperature. Materials exhibiting PTC behavior are useful in a number of applications including electrical circuit protection devices in which the current passing through a circuit is controlled by the temperature of a PTC element forming part of that circuit.
Particularly useful devices comprising current limiting polymer compositions are electrical circuit protection devices. Such circuit protection devices usually contain a current limiting polymer device comprised of two electrodes embedded in a current limiting polymer composition. When connected to a circuit, the circuit protection devices have a relatively low resistance under normal operating conditions of the circuit, but are tripped, that is, converted into a high resistance state when a fault condition, for example, excessive current or temperature, occurs. When the circuit protection device is tripped by excessive current, the current passing through the PTC device causes it to self-heat to its switching temperature, Ts, at which a rapid increase in its resistance takes place.
U.S. Pat. No. 5,841,111 (Shea, et al.) describes such devices and lists, very generally, as a suggested, useful polymer matrix, all of: polyethylene or propylene copolymers, polyvinyl chloride or fluoride, polyvinylidene chloride or fluoride, polyesters such as poly (ethylene terephthalate), polyamides such as various Nylons, polystyrene polyacrylonitrile, thermoplastic silicones or polyethers, rubbers, elastomeric gums such as polyisoprene, styrene-butadiene random copolymer rubbers, polyacrylate rubbers, siloxanes such as poly (dimethyl siloxane), ethylene-propylene/diene rubbers, epoxy resins made from epichlorohydrin and bisphenol A or epichlorohydrin and aliphatic polyols such as glycerol all usually with amide or amine curing agents, and phenolic resins made from a phenol and an aldehyde; essentially the entire range of resinous materials. Similarly, U.S. Pat. No. 5,614,881 (Duggal, et al.) teaches use of almost any resin in a current limiting device. Other patents describing conductive polymer compositions include U.S. Pat. Nos. 4,772,422 and 5,250,228 (Hijikata, et al. and Baigrie et al. respectively).
U.S. Pat. No. 5,859,578 (Arnold), relating specifically to circuit breakers, describes the polymer current limiter as conductive filler and any polymeric binder with a vaporization temperature such that significant gas evolution occurs below 800xc2x0 C. However, most previous commercial materials used for current limiting applications in conjunction with low voltage ( less than 600 Vac) circuit breakers, generally consisted of a very brittle blend of conductive filler (that is, carbon black) with a binder that was difficult to process, especially to mold. Parts had to be molded to the finished dimensions since secondary operations, such as cutting or sanding, generally resulted in breakage. This reason alone, caused a high cost per part. Also, these brittle materials tended to crumble with time and tended to have limited application due to the high let-through values, that is, only moderate short circuit performance.
While many patents expound a wide variety of current limiting plastics, the usual actual commercial material is carbon black in a matrix of polyethylene thermoplastic. This material is not only brittle, but also tends to oxidize over time, even if antioxidants have been added to the polymer. The high operating temperatures of the environment where current limiting polymers are used additionally accelerates the oxidation of the polyethylene. Polyethylene is used as the commercial matrix material, in part, because the PTC effect on many other materials is not clear, contrary to many patent teachings which suggest any resin is appropriate. J. Fournier et al. in Jour. Materials Science Letters xe2x80x9cPositive temperature coefficient effect in carbon black/epoxy polymer compositesxe2x80x9d 16 (1997) pp 1677-1679, describe carbon black/polyethylene PTC materials, the principle for the PTC effect of a carbon black/epoxy resin prepolymer matrix. The epoxy resin used was the diglycidyl ether of bisphenol A (xe2x80x9cDGEBAxe2x80x9d, specifically DER 332, sold commercially by Dow Chemical) with either isophorene diamine (xe2x80x9cIPDxe2x80x9d) or triethylene tetramine (xe2x80x9cTETAxe2x80x9d) hardeners. They found that 18-20 wt. % carbon black provided the best PTC effect. But that such effect was difficult to predict because of the small thermal expansion of the matrix when heated. The nature of the carbon particles in the epoxy system was also thought to strongly influence the PTC effect. R. Strumpler et al. in Materials Research Society Symp. Proc. xe2x80x9cCorrelation of Electrical, Dielectric and Mechanical Properties of Polymer Compositesxe2x80x9d Vol. 411, 1996 pp. 393-398 describe problems with PTC behavior of metallic particle filled epoxies due to the shrinkage of the epoxy during cross-linking and the build up of internal stresses that result in a lower interparticle contact resistance.
Much of the lack of predictability in conductive polymer blends stems from the lack of understanding of the mechanism(s) responsible for the PTC effect. Material type, combination, and process parameters have been shown to greatly influence the PTC effect. The predominant theory for the PTC effect is thermal expansion of the polymer matrix. However, there are many examples, Fournier et al. and Strumpler et al., showing that PTC effect does not correlate with thermal expansion of the polymer. This is explained by the type of hardness and particle shape/size of the conductive filler as well as the internal stress built-up in the polymer during the molding process. All these factors, plus other unknowns, lead to the current lack of predictability when explaining this class of materials. However, the material characteristics can be reliably reproduced.
Such problems with epoxy resins may be why Sadhir et al., in PCT Publication WO 99/30329, teach intermixing discretely distributed thermoplastic material and conductive material in an epoxy matrix, where the epoxy and thermoplastic are substantially immiscible, providing thermally stable compositions exhibiting PTC behavior. The preferred materials therefor are a mixture of crystalline, oxidized polyethylene and an epoxy resin combination such as a diglycidylether of bisphenol A having an epoxy equivalent weight of 170-180 (DER-332 sold commercially by Dow Chemical) and a diglycidylether of neopentylglycol, crosslinked by a borontrifluoride complex, dicyanodiamide, or preferably, an anhydride. This, however, provides a fairly complicated system requiring careful matching of ingredients, although it does utilize, in part, epoxy resins, which are known to have outstanding electrical and molding characteristics.
What is needed, however, are current limiting polymers that are a mixture of readibly commercially available materials, such as epoxy resins, that exhibit a PTC effect and that are flexible and moldable, can be finished, are not brittle upon cure, and that are cuttable or punchable so they can be inexpensively volume produced in long sheet form.
Therefore, it is one of the main objects of this invention to provide an epoxy based current-limiting material which is moldable, and not brittle upon cure so that it can be finished if necessary.
It is also one of the main objects of this invention to provide an epoxy based current-limiting material which can be cast as a thin film (about 40 cmxc3x9780 cm and between 0.05 cm and 0.5 cm, usually 0.13 cm (0.05 inch) thick, and then cut into smaller component pieces for example 6.1xc3x974.0xc3x970.12 cm thick (2.4 inchxc3x971.6 inchxc3x970.05 inch) without fracturing.
These and other objects of the invention are accomplished by providing an electrically conducting material exhibiting superior flexibility and punchability, electrical conductivity characteristics; and low let-through (that is, the measure of effectiveness of the current limiter in reducing current and the duration of the current, typically less than 10xc3x97103 A2 sec), for use in a current limiting PTC polymer device, consisting essentially of the cured reaction product of: a resin component comprising a mixture of: 100 parts by weight of a short chain aliphatic diepoxide resin and 0 to 15 parts by weight of a bisphenol A epoxy resin, 80 to 150 parts by weight of conductive filler, and curing agent. Preferably the aliphatic diepoxide is the diglycidyl ether of an alkylene glycololigomer, the bisphenol A epoxy resin is present in the range of 1 to 10 parts by weight to add strength to the material, and the curing agent is a borontrifluoride-amine complex. In some instances when no epoxidized bisphenol A epoxy is present a minor amount, about 2 to 20 parts by weight, of an epoxidized polybutadiene may be present.
This new material overcomes many problems associated with the previous state-of-the-art materials. This new flexible material greatly reduces manufacturing costs, has a significantly better short-circuit performance, and has a greater lifetime than previous materials used in high power current-limiting applications. Lowering the polymer vaporization temperature produced faster switching times resulting in decreased let-through. Also, because of the conformability of the new flexible polymer with the electrodes, let-through values have been reduced. Thus, it is possible to achieve the desired low device resistance with reduced forces on the electrodes due to the new material properties, which translate to lower let-through values.
Increased xe2x80x9cflexibilityxe2x80x9d, that is, the ability to be cut, pressed, punched or the like after final cure without fracture, crumbling or breaking, as, for example, punching 1 cmxc3x972 cmxc3x97xc2xd cm thickness pieces from a thin sheet 26 cmxc3x9726 cmxc3x97xc2xd cm thickness without cracking are achievable. Also, let-through values below 10xc3x97103 (Amp2 sec), in devices rated for 1.94 A/cm2 are achievable, as well as all the other epoxy resin electrical and physical virtues well known in the art. The term xe2x80x9cpunchabilityxe2x80x9d is to be equated with cuttability and both are related to the xe2x80x9cflexibilityxe2x80x9d of the material. The invention also resides in a current limiting PTC polymer device comprising at least two electrodes and the previously described resin/filler/curing agent combination therebetween in contacting relationship.